1. Field of the Invention
The invention relates to the thermal production of activated carbon and other heat-treated carbons.
2. Description of the Related Art
Recent environmental awareness has focused on the need to remove harmful mercury emissions from coal fired power plants. These efforts have led to several developments with regards to effective mercury removal techniques. The most successful method for mercury removal utilizes pulverized activated carbon injected into the flue gas stream of a coal fired power plant. Extensive research in this field has demonstrated that not all activated carbons effectively remove mercury. There are many factors that influence mercury removal effectiveness using activated carbon in coal fired power plants such as coal fuel type, mercury concentrations, etc. One of the largest factors in the ability of activated carbon to adsorb mercury is the activated carbon pore structure. Not all activated carbons have a suitable pore structure and other characteristics required in order to be effective. Often activated carbons are treated with various agents or gases to improve mercury oxidation and removal. Regardless of whether or not the activated carbon is treated with an enhancing agent, invariably the activated carbons used for this application are selected because of specific physical characteristics considered essential for effective for mercury removal. Based on this relatively successful approach to reducing mercury emissions, so-called activated carbon (treated and untreated) is thought to provide very efficient reduction.
Activated carbon is a term used to describe a carbon material that has been modified to possess a very high surface area that is useful for adsorption, deodorization, and other applications. Thus, activated carbon (AC) refers to carbon that has had its pore structure opened or created. Activated carbon can be produced in two ways. The first is thermal activation where carbon containing material, such as coal, becomes activated by heating it with steam and/or other activating agents such as CO2. The second activation process uses various chemicals to create the open pore structure. These treatments remove residual non-carbon elements and produce a porous internal microstructure having an extremely high surface area. A single grain of such material can have 400 to 1200 square meters of surface area, comprising up to 98% of it internal structure.
Pore structure has several classifications: Micro-pores (<1 mm), Mesa-pores (1 to 25 nm) and Macro-pores (>25 nm). Mesa-pore AC is well suited for mercury adsorption. AC is also classified by its particulate size range. Generally AC in powdered form of 50 mesh and finer particulate size is referred to as pulverized activated carbon (PAC) and the granular form of 4 to 50 mesh particulates is referred to as granular activated carbon (GAC).
As previously stated, carbon can be thermally or chemically activated. Chemical activation can be considerably more costly and thermal activation is the current preferred method for producing AC suitable for mercury removal. Thermal AC production methods include rotary hearth furnaces, rotary calciners, and various other forms of calciners and other heat-treatment apparatuses. One of the most common methods of AC product (FIG. 1) can be characterized by two processing stages. The first stage is composed of thermal devolatilization, decomposition, or carbonization of the carbonaceous feed material. The second stage is the gasification or activation of the carbonized char material. Though these stages imply that devolatilization and activation are separate reactions they do in reality overlap to a large degree depending on process conditions. A portion of the carbonaceous feed is invariably activated during devolatilization. Likewise a portion of the carbonaceous feed is further or more completely devolatilized during activation.
In the devolatilization step, moisture, hydrogen and oxygen are removed from the carbonaceous feed material to open existing pore structure in the carbonaceous feed. During activation, oxidizing gases such as steam, CO2, or oxygen is used to complete devolatilization and create new additional pore structure through partial or selective gasification of carbon in the devolatilized feed. It is well documented that activation by definition is a selective gasification reaction. The terms activation, gasification and partial combustion or oxidation are very closely related and in many cases have overlapping meanings. Thermal activation is most often accomplished in direct fire rotary kilns or multi-hearth furnaces, often reaching temperatures greater than 1000 degrees Centigrade.
While thermal activation is the most widely used method of AC production and has a long and proven track record, operational and capital cost remain high. The cost of existing thermal activation methods is considerable due to the current cost of capital, energy, emissions control and waste disposal. Indeed, devolatilization and activation of material with carbon content typically is thermally treated at temperatures in the general range of 600-1200° (1112-2192° F.) over long periods of time and in multiple stages. The time required in each stage can range from minutes to hours.
Newer concepts for AC production have emerged in which AC is produced in a single reaction vessel through what the inventor terms as “flash activation” processes which refers to any process scheme where devolatilization and subsequent activation reactions require only seconds to complete. These flash activation processes use the principle of rapid devolatilization with heat, moisture, and other oxidizers. Such methods results in varying degrees of concurrent char formation and activation commonly found in partial combustion reactions, coal gasification, and other similar devolatilization schemes.
Regardless of the scheme used for flash activation, carbonaceous feed, air, heat, and oxidizers such as CO2, O2 and moisture, are reacted in a gaseous environment. This reaction produces suitable conditions for devolatilization and activation reactions. Many calciners and other heat-treatment vessels could be operated to produce an activated char product of varying degrees of activation using the flash activation principle. For example the KBR Transport Reaction Vessel Gasifier is a known method of coal gasification. Such a method would produce an activated carbon if the process conditions were altered to favor partial gasification of the carbonaceous feed. Therefore the principle of rapid and concurrent devolatilization and activation (i.e., flash activation) is not in itself unique. It is the quality of the produced AC and the successful commercial adaptation of this principle which are the most important factors.
Some examples of the adaptation of this principle called by various different names include the “Thief Method for Production of Activated Carbon,” the “Sorbent Activation Process (SAP).” and the “PraxAir Hot Oxygen Burner (HOB) PAC Production Method” among others. These methods tie this fundamental devolatilization and activation principle to specific apparatuses such as a boiler, coal power plant flue gas flow, or burner design. However, these methods have not yet demonstrated commercial production capability and do not produce an AC with comparable yield, composition, and overall quality as traditional thermal AC production methods. This is due in part to the general over simplification the complex process variables and reaction vessel design. The prior art does not teach effective reaction vessel design required to produce the optimal reaction vessel thermal, particulate flow, and oxidizing and reducing condition profile necessary to better control process reaction conditions. Furthermore, it often is difficult to control the inadvertent loss of carbon through excessive gasification reactions in heretofore known thermal flash activation processes. The excessive loss of carbon reduces the product yield raising production costs considerably and greatly increases the residual ash content thereby further diminishing the AC product quality. Therefore, the consistent production of the highest quality AC with good product yield (and/or AC of unique or different activation characteristics and applications) remains elusive, particularly on industrial scales of production.
Thus, there is a need for a rapid and less costly way to produce activated carbon of high quality and/or different activation characteristics and that allows for more precise and versatile controlling of the devolatilization and activation process conditions.